Light emitting diodes (LEDs) are being used as light sources in an increasing variety of applications extending from communications and instrumentation to household, automotive and visual display uses. In many of these applications color variability is advantageous or required. LEDs have great promise for color variable applications because of their rapid switching time, small size, high reliability, long life and simplicity of color control. A common color variable LED assembly combines red, green and blue (RGB) LEDs in an RGB assembly. Color can be controlled and changed by varying the outputs of each different color LED.
Unfortunately, conventional multi-colored LEDs, including conventional RGB assemblies, suffer from poor color mixing. Because the LED die emit considerable heat, the LED die are typically widely spaced for heat dissipation. As a consequence, die of different color are spaced apart, and viewers may see different individual colors from different viewing angles.
FIG. 1, which is a schematic cross-section of a conventional RGB assembly 10, is useful in understanding poor color mixing. The assembly 10 includes red, green and blue LED die 11R, 11G and 11B, respectively disposed on a mounting base 12 and encapsulated in a transparent dome 13. The die can be mounted within a surface cavity (not shown) in the base 12. In the particular arrangement illustrated, green light from die 11G dominates viewing angles on the left side of the assembly, blue light from 11B dominates views from the right side and red light from 11R dominates central viewing. There is both poor illuminance overlap and poor luminous intensity overlap of the respective colors. Illuminance is defined as the total light flux incident on a surface, per unit area, as perceived by the human eye. Luminous intensity is defined as a measure of the wavelength-weighted power emitted by a light source in a particular direction, based on a standardized model of the sensitivity of the human eye.
Attempts to improve color mixing in the prior art include externally attached secondary optics. Such devices include fly's eye lenslet arrays, holographic diffusers, microlens patterns, and scatter devices. These devices may be attached to refractors, reflectors or hybrid collimators. Other such attachments include internal reflection (TIR) integrators and mirror integrators, which improve overlap of colors at their respective exit apertures.
All such secondary optic devices suffer from several disadvantages. External devices may be bulky and unwieldy for integrated luminaire applications. Secondary optic devices may improve color mixing by spreading the light into a wider beam, but as the light is spread, luminous intensity drops according to etendue and skewness invariance laws. Etendue is a property of an optical system, which characterizes how “spread out” the light is in area and angle. This is undesirable for high intensity narrow distribution applications. TIR or mirror integrators may bring some or all of the luminous intensity back to a full overlap at a given distance from the illumination sources, but suffer from poor matching at other distances from the source. For certain types of TIR mirror integrators, the illuminance distribution functions of different color sources may overlap well, but their corresponding luminous intensity distribution functions do not overlap well. In such circumstances, the colors again separate at further distances from the sources. In other situations, the luminous intensity distributions may overlap well, but the illuminance distributions do not, thereby resulting in one color being visible near the center of the plane at which the sources are viewed, while other colors are more visible near the fringes of the plane, with the resulting illuminance distribution having the appearance of a “bull's eye.”
Thus, it is desirable for the light produced by a multi-color array to create a more uniform color mixture everywhere within the emitted light beam. Accordingly, there is a need for a multicolor LED assembly with improved and controlled color mixing.